As described in previous reports, a hemagglutination inhibition test suitable for the diagnosis of pregnancy in macaques has been developed in this laboratory. The Subhuman Primate Pregnancy Test kit, distributed under the aegis of the Contraceptive Development Branch, Center for Population Research, NICHD, has been sent to primate facilities all over the world. To date, more than 500 kits have been delivered, sufficient for 25,000 pregnancy tests. In order to realize the full potential of this kit, we have undertaken a series of studies to determine the interval for most reliable detection of urinary chorionic gonadotropin in baboons, marmosets, chimpanzees and squirrel monkeys. The first 3 of these are now complete.